(1) Field of the Invention
This invention relates to methods of forming colored patterns on the surfaces of aluminum or its alloys without using such dyes and pigments as hitherto being used for coloring of aluminum or its alloys.
(2) Description of the Prior Art
Hitherto, various methods have been known in the art for forming colored patterns on the surfaces of aluminum or its alloys. In prior methods, as disclosed in Japanese Patent Publication Nos. 4616/75 and 3895/77 and Japanese Patent Application laid open to public inspection No. 41735/77, a resist film is applied to or printed on an aluminum work piece to form protected areas in confirmity with a desired pattern, the work piece is then subjected to an anodic oxidation to form thereon a barrier-type oxide film or to a chemical conversion to form a chemically oxidized film and, after removal of the resist film, to the second anodic oxidation or chemical conversion to form a pattern of a colored film, namely, the prior methods include the steps of: resist pattern printing, primary anodic oxidation, (stopping-up of pores), removal of resist films, secondary anodic oxidation (electrolytic coloring); or, resist pattern printing, chemical oxide film formation, removal of resist film and chemical conversion (chemical formation of colored oxide film). Another method which also includes a printing step for patterning is disclosed in Japanese Patent Publication No. 21022/76 which comprises applying a TFS coating to the surface of an aluminum work piece, applying a pattern coating thereon by means of screen printing or off-set printing and dryng and baking together the TFS coating and pattern coating. These prior methods including essentially a printing process for patterning have a shortcoming that the printing process is expensive and takes much time and, consequently, results in decrease in the mass productivity or productivity of the methods and, in addition, the printing process makes it difficult to produce a great variety of patterns each at relatively small produce.
A method of forming patterns on aluminum or its alloys without employing any printing process is disclosed in Japanese Patent Application laid open to public inspection No. 60244/77 which comprises subjecting an aluminum work piece to electrolysis in an alkaline electrolytic bath added with a barrier-type oxide film forming electrolyte by means of an alternating current or current exhibiting the same effect with alternating current. Japanese Patent Application laid open to public inspection Nos. 3535/77, 61139/77 and 70951/77 disclose methods of forming colored patterns on aluminum surfaces by electrolytic coloring through control of electrolytic formation of a barrier-layer after anodic oxidation. However, these methods in which colored patterns are formed by electrolytic coloring after modification of the thickness of the barrier-layer are unsuitable for work pieces having complicated shapes and poor in productivity because of difficulty in modification of the thickness of the barrier-layer. On the other hand, the method as disclosed in Japanese Patent Application laid open to public inspection No. 60244/77 is applicable to work pieces having complicated shapes, though the patterns formed in this method are lengthwise extending short etching figures which are somewhat similar to but far apart from the straight grain of natural wood and it is impossible to form pattern imitating the cross grain of wood.